The toner used in electrographic copiers and printers is a blend of particles, including plastic resins, coloring pigments and other ingredients. Most toners are manufactured in bulk using a melt mixing or hot compounding process. Plastic resins, carbon black, magnetic iron oxides, waxes and charge control agents are blended together while in a molten state to thereby form a hot paste having a consistency similar to cake mix. This mixture is then cooled, typically by forming it into slabs on a cooling belt or by pelletizing the mixture and cooling the pellets. The raw toner is then ground or pulverized into a toner powder by jet mills or air-swept hammer mills. This process produces a powder having a wide range of particle sizes. The toner powder is sifted to remove over-size and under-size toner particles. The pulverized, sifted toner powder is then blended with additives to adjust flow and electrostatic properties. The finished toner powder has particle sizes that range from, for example, twelve microns (μ) to approximately eight microns and smaller. The bulk toner is typically placed into large-sized or bulk containers, such as, for example, large barrels.
The toner powder is typically repackaged from the large bulk containers into smaller intermediate or end-use containers that are suitable for sale to and/or use by end users. Repackaging the toner from the bulk containers into smaller containers generally involves gravity-assisted flow of the toner from the bulk container into a sieve, such as a vibratory sieve, and into the smaller containers. The sieve typically contains a mesh or screen filter through which the toner powder must flow. The filter is intended to prevent the passage of agglomerated toner particles and contaminants into the smaller containers. The mesh or screen filter has very fine openings, such as, for example, from approximately 200 to approximately 400 openings per inch, and is typically constructed of a metal, such as, for example, stainless steel.
The fine mesh filter occasionally becomes clogged or blinded due to an accumulation of agglomerated toner powder, oversized toner, and/or foreign particles thereon. The partial or complete clogging or blinding of the filter significantly reduces or stops the throughput of product through the screen, and a build-up of toner powder above the screen results. The weight of the built-up of toner powder bears directly upon the fine screen and may result in tearing of the screen. When a screen tears, the coarse material collected thereon is undesirably conveyed through the sieve thereby contaminating otherwise acceptable product. The contaminated product must be recycled, i.e., re-processed through the sieve.
In order to prevent the above-described overloading and tearing of sieve screens, the sieves must be shut down and preventive maintenance and cleaning of the screens performed. The performance of such preventative maintenance, and the resulting down time of the sieves, is costly and inefficient. Further, the preventive maintenance must be performed on a predicted minimum schedule, which may often be premature for a particular screen, thereby causing unnecessary down time of the sieves.
Therefore, what is needed in the art is a method and apparatus to detect a build-up of powder on the screen.
Further, what is needed in the art is a method and apparatus for detecting a blinded or clogged screen thereby indicating the need for cleaning and/or preventive maintenance.
Moreover, what is needed in the art is a method and apparatus that prevents overloading and/or tearing of filter screens, and which increases the useful life of a filter screen.